In practice, the most common usage encountered in academic and popular writing describes land animals roughly larger than a human that are not (solely) domesticated. The term is especially associated with the Pleistocene megafauna – the land animals often larger than modern counterparts considered archetypical of the last ice age, such as mammoths, the majority of which in northern Eurasia, the Americas and Australia became extinct as recently as 10,000–40,000 years ago.[4] It is also commonly used for the largest extant wild land animals, especially elephants, giraffes, hippopotamuses, rhinoceroses, and large bovines. Megafauna may be subcategorized by their trophic position into megaherbivores (e.g., elk), megacarnivores (e.g., lions), and, more rarely, megaomnivores (e.g., bears).

Other common uses are for giant aquatic species, especially whales, any larger wild or domesticated land animals such as larger antelope and cattle, as well as numerous dinosaurs and other extinct giant reptilians.

The term is also sometimes applied to animals (usually extinct) of great size relative to a more common or surviving type of the animal, for example the 1 m (3 ft) dragonflies of the Carboniferous period.

Megafauna – in the sense of the largest mammals and birds – are generally K-strategists, with high longevity, slow population growth rates, low mortality rates, and (at least for the largest) few or no natural predators capable of killing adults. These characteristics, although not exclusive to such megafauna, make them vulnerable to human overexploitation, in part because of their slow population recovery rates.

One observation that has been made about the evolution of larger body size is that rapid rates of increase that are often seen over relatively short time intervals are not sustainable over much longer time periods. In an examination of mammal body mass changes over time, the maximum increase possible in a given time interval was found to scale with the interval length raised to the 0.25 power.[5] This is thought to reflect the emergence, during a trend of increasing maximum body size, of a series of anatomical, physiological, environmental, genetic and other constraints that must be overcome by evolutionary innovations before further size increases are possible. A strikingly faster rate of change was found for large decreases in body mass, such as may be associated with the phenomenon of insular dwarfism. When normalized to generation length, the maximum rate of body mass decrease was found to be over 30 times greater than the maximum rate of body mass increase for a ten-fold change.[5]

Subsequent to the Cretaceous–Paleogene extinction event that eliminated the non-avian dinosaurs about 66 Ma ago, terrestrial mammals underwent a nearly exponential increase in body size as they diversified to occupy the ecological niches left vacant.[6] Starting from just a few kg before the event, maximum size had reached ~50 kg a few million years later, and ~750 kg by the end of the Paleocene. This trend of increasing body mass appears to level off about 40 Ma ago (in the late Eocene), suggesting that physiological or ecological constraints had been reached, after an increase in body mass of over three orders of magnitude.[6] However, when considered from the standpoint of rate of size increase per generation, the exponential increase is found to have continued until the appearance of Indricotherium 30 Ma ago. (Since generation time scales with body mass0.259, increasing generation times with increasing size cause the log mass vs. time plot to curve downward from a linear fit.)[5]

Megaherbivores eventually attained a body mass of over 10 000 kg. The largest of these, indricotheres and proboscids, have been hindgut fermenters, which are believed to have an advantage over foregut fermenters in terms of being able to accelerate gastrointestinal transit in order to accommodate very large food intakes.[7] A similar trend emerges when rates of increase of maximum body mass per generation for different mammalian clades are compared (using rates averaged over macroevolutionary time scales). Among terrestrial mammals, the fastest rates of increase of body mass0.259 vs. time (in Ma) occurred in perissodactyls (a slope of 2.1), followed by rodents (1.2) and proboscids (1.1),[5] all of which are hindgut fermenters. The rate of increase for artiodactyls (0.74) was about a third that of perissodactyls. The rate for carnivorans (0.65) was slightly lower yet, while primates, perhaps constrained by their arboreal habits, had the lowest rate (0.39) among the mammalian groups studied.[5]

Terrestrial mammalian carnivores from several eutherian groups (the mesonychidAndrewsarchus, the creodontsMegistotherium and Sarkastodon, and the carnivorans Amphicyon and Arctodus) all reached a maximum size of about 1000 kg[6] (the carnivoran Arctotherium apparently actually got somewhat larger). The largest known metatherian carnivore, Proborhyaena gigantea, apparently reached 600 kg, also close to this limit.[8] A similar theoretical maximum size for mammalian carnivores has been predicted based on the metabolic rate of mammals, the energetic cost of obtaining prey, and the maximum estimated rate coefficient of prey intake.[9] It has also been suggested that maximum size for mammalian carnivores is constrained by the stress the humerus can withstand at top running speed.[8]

Analysis of the variation of maximum body size over the last 40 Ma suggests that decreasing temperature and increasing continental land area are associated with increasing maximum body size. The former correlation would be consistent with Bergmann's rule,[10] and might be related to the thermoregulatory advantage of large body mass in cool climates,[6] better ability of larger organisms to cope with seasonality in food supply,[10] or other factors;[10] the latter correlation could be explainable in terms of range and resource limitations.[6] However, the two parameters are interrelated (due to sea level drops accompanying increased glaciation), making the driver of the trends in maximum size more difficult to identify.[6]

Since tetrapods (first reptiles, later mammals) returned to the sea in the Late Permian, they have dominated the top end of the marine body size range, due to the more efficient intake of oxygen possible using lungs.[11][12] The ancestors of cetaceans are believed to have been the semiaquatic pakicetids, no larger than wolves, of about 53 million years (Ma) ago.[13] By 40 Ma ago, cetaceans had attained a length of 20 m or more in Basilosaurus, an elongated, serpentine whale that differed from modern whales in many respects and was not ancestral to them. Following this, the evolution of large body size in cetaceans appears to have come to a temporary halt, and then to have backtracked, although the available fossil records are limited. However, in the period from 31 Ma ago (in the Oligocene) to the present, cetaceans underwent a significantly more rapid sustained increase in body mass (a rate of increase in body mass0.259 of a factor of 3.2 per million years) than achieved by any group of terrestrial mammals.[5] This trend led to the largest animal of all time, the modern blue whale. Several reasons for the more rapid evolution of large body size in cetaceans are possible. Fewer biomechanical constraints on increases in body size may be associated with suspension in water as opposed to standing against the force of gravity, and with swimming movements as opposed to terrestrial locomotion. Also, the greater heat capacity and thermal conductivity of water compared to air may increase the thermoregulatory advantage of large body size in marine endotherms, although diminishing returns apply.[5]

Cetaceans are not the only marine mammals to reach unprecedented size in the modern era. The largest carnivoran of all time is the mostly aquatic modern southern elephant seal.

Because of the small initial size of all mammals following the extinction of the dinosaurs, nonmammalian vertebrates had a roughly ten-million-year-long window of opportunity (during the Paleocene) for evolution of gigantism without much competition.[14] During this interval, apex predator niches were often occupied by reptiles, such as terrestrial crocodilians (e.g. Pristichampsus), large snakes (e.g. Titanoboa) or varanid lizards, or by flightless birds[6] (e.g. Gastornis in Europe and North America, Paleopsilopterus in South America). This is also the period when flightless herbivorous paleognath birds evolved to large size on a number of Gondwanan land masses. These birds, termed ratites, have traditionally been viewed as representing a lineage separate from that of their small flighted relatives, the Neotropictinamous. However, recent genetic studies have found that tinamous nest well within the ratite tree, and are the sister group of the extinct moa of New Zealand.[15][14][16] Similarly, the small kiwi of New Zealand have been found to be the sister group of the extinct elephant birds of Madagascar.[14] These findings indicate that flightlessness and gigantism arose independently multiple times among ratites via parallel evolution.

In the northern continents, large predatory birds were displaced when large eutherian carnivores evolved. In isolated South America, the phorusrhacids could not be outcompeted by the local metatherian sparassodonts and remained dominant until advanced eutherian predators arrived from North America (as part of the Great American Interchange) during the Pliocene. However, none of the largest predatory (Brontornis), possibly omnivorous (Dromornis[17]) or herbivorous (Aepyornis) flightless birds of the Cenozoic ever grew to masses much above 500 kg, and thus never attained the size of the largest mammalian carnivores, let alone that of the largest mammalian herbivores. It has been suggested that the increasing thickness of avian eggshells in proportion to egg mass with increasing egg size places an upper limit on the size of birds.[18][note 1] The largest species of Dromornis, D. stirtoni, may have gone extinct after it attained the maximum avian body mass and was then outcompeted by marsupial diprotodonts that evolved to sizes several times larger.[21]

An analysis of Sporormiella fungal spores (which derive mainly from the dung of megaherbivores) in swamp sediment cores spanning the last 130,000 years from Lynch's Crater in Queensland, Australia showed that the megafauna of that region virtually disappeared about 41,000 years ago, at a time when climate changes were minimal; the change was accompanied by an increase in charcoal, and was followed by a transition from rainforest to fire-tolerant sclerophyll vegetation. The high-resolution chronology of the changes supports the hypothesis that human hunting alone eliminated the megafauna, and that the subsequent change in flora was most likely a consequence of the elimination of browsers and an increase in fire.[42][43][44] The increase in fire lagged the disappearance of megafauna by about a century, and most likely resulted from accumulation of fuel once browsing stopped. Over the next several centuries grass increased; sclerophyll vegetation increased with a lag of another century, and a sclerophyll forest developed after about another thousand years.[44] During two periods of climate change about 120 and 75 thousand years ago, sclerophyll vegetation had also increased at the site in response to a shift to cooler, drier conditions; neither of these episodes had a significant impact on megafaunal abundance.[44] Similar conclusions regarding the culpability of human hunters in the disappearance of Pleistocene megafauna were obtained via an analysis of a large collection of eggshell fragments of the flightless Australian bird Genyornis newtoni[45] and from analysis of Sporormiella fungal spores from a lake in eastern North America.[46][47]

A number of other mass extinctions occurred earlier in Earth's geologic history, in which some or all of the megafauna of the time also died out. Famously, in the Cretaceous–Paleogene extinction event the dinosaurs and most other giant reptilians were eliminated. However, the earlier mass extinctions were more global and not so selective for megafauna; i.e., many species of other types, including plants, marine invertebrates[48] and plankton, went extinct as well. Thus, the earlier events must have been caused by more generalized types of disturbances to the biosphere.

Megafauna play a significant role in the lateral transport of mineral nutrients in an ecosystem, tending to translocate them from areas of high to those of lower abundance. They do so by their movement between the time they consume the nutrient and the time they release it through elimination (or, to a much lesser extent, through decomposition after death).[49] In South America's Amazon Basin, it is estimated that such lateral diffusion was reduced over 98% following the megafaunal extinctions that occurred roughly 12,500 years ago.[50][51] Given that phosphorus availability is thought to limit productivity in much of the region, the decrease in its transport from the western part of the basin and from floodplains (both of which derive their supply from the uplift of the Andes) to other areas is thought to have significantly impacted the region's ecology, and the effects may not yet have reached their limits.[51]

Large populations of megaherbivores have the potential to contribute greatly to the atmospheric concentration of methane, which is an important greenhouse gas. Modern ruminantherbivores produce methane as a byproduct of foregut fermentation in digestion, and release it through belching. Today, around 20% of annual methane emissions come from livestock methane release. In the Mesozoic, it has been estimated that sauropods could have emitted 520 million tons of methane to the atmosphere annually,[52] contributing to the warmer climate of the time (up to 10 C warmer than at present).[52][53] This large emission follows from the enormous estimated biomass of sauropods, and because methane production of individual herbivores is believed to be almost proportional to their mass.[52]

Recent studies have indicated that the extinction of megafaunal herbivores may have caused a reduction in atmospheric methane. This hypothesis is relatively new.[54] One study examined the methane emissions from the bison that occupied the Great Plains of North America before contact with European settlers. The study estimated that the removal of the bison caused a decrease of as much as 2.2 million tons per year.[55] Another study examined the change in the methane concentration in the atmosphere at the end of the Pleistocene epoch after the extinction of megafauna in the Americas. After early humans migrated to the Americas about 13,000 BP, their hunting and other associated ecological impacts led to the extinction of many megafaunal species there. Calculations suggest that this extinction decreased methane production by about 9.6 million tons per year. This suggests that the absence of megafaunal methane emissions may have contributed to the abrupt climatic cooling at the onset of the Younger Dryas.[54] The decrease in atmospheric methane that occurred at that time, as recorded in ice cores, was 2-4 times more rapid than any other decrease in the last half million years, suggesting that an unusual mechanism was at work.[54]

The red kangaroo (Macropus rufus) is the largest living Australian mammal and marsupial at a weight of up to 85 kg (187 lb). However, its extinct relative, the giant short-faced kangarooProcoptodon goliah reached 230 kg (510 lb), while extinct diprotodonts attained the largest size of any marsupial in history, up to an estimated 2,750 kg (6,060 lb). The extinct marsupial lion (Thylacleo carnifex), at up to 160 kg (350 lb) was much larger than any extant carnivorous marsupial.

Elephants are the largest living land animals. They and their relatives arose in Africa, but until recently had a nearly worldwide distribution. The African bush elephant (Loxodonta africana) has a shoulder height of up to 4.3 m (14 ft) and weighs up to 13 tons. Among recently extinct proboscideans, mammoths (Mammuthus) were close relatives of elephants, while mastodons (Mammut) were much more distantly related. The steppe mammoth (M. trogontherii) is estimated to have commonly weighed around 10 tonnes, making it possibly the largest proboscid, which would make it the second largest land mammal after indricotherines.

The largest sirenian at up to 1500 kg is the West Indian manatee (Trichechus manatus). Steller's sea cow (Hydrodamalis gigas) was probably around five times as massive, but was exterminated by humans within 27 years of its discovery off the remote Commander Islands in 1741. In prehistoric times this sea cow also lived along the coasts of northeastern Asia and northwestern North America; it was apparently eliminated from these more accessible locations by aboriginal hunters.

Ground sloths were another group of slow, terrestrial xenarthrans, related to modern tree sloths. They had a similar history, although they reached North America earlier, and spread farther north (e.g., Megalonyx). The largest genera, Megatherium and Eremotherium, reached sizes comparable to elephants.[56]

The extant capybara (Hydrochoerus hydrochaeris) of South America, the largest living rodent, weighs up to 65 kg (143 lb). Several recently extinct North American forms were larger: the capybara Neochoerus pinckneyi (another neotropic migrant) was about 40% heavier; the giant beaver (Castoroides ohioensis) was similar. The extinct blunt-toothed giant hutia (Amblyrhiza inundata) of several Caribbean islands may have been larger still. However, several million years ago South America harbored much more massive rodents. Phoberomys pattersoni, known from a nearly full skeleton, probably reached 700 kg (1,500 lb). Fragmentary remains suggest that Josephoartigasia monesi grew to upwards of 1,000 kg (2,200 lb).

Bears are large carnivorans of the caniform suborder. The largest living forms are the polar bear (Ursus maritimus), with a body weight of up to 680 kg (1,500 lb), and the similarly sized Kodiak bear (Ursus arctos middendorffi), again consistent with Bergmann's rule. Arctotherium augustans, an extinct short-faced bear from South America, was the largest predatory land mammal ever with an estimated average weight of 1,600 kg (3,500 lb).[61]

Tapirs are browsing animals, with a short prehensile snout and pig-like form that appears to have changed little in 20 million years. They inhabit tropical forests of Southeast Asia and South and Central America, and include the largest surviving land animals of the latter two regions. There are four species.

Extinct dromornithids of Australia such as Dromornis may have exceeded the largest ratites in size. (Due to its small size for a continent and its isolation, Australia is sometimes viewed as the world's largest island; thus, these species could also be considered insular giants.)

The Permian temnospondyl Prionosuchus, the largest amphibian known, reached 9 m in length and was an aquatic predator resembling a crocodilian. After the appearance of real crocodilians, temnospondyls such as Koolasuchus (5 m long) had retreated to the Antarctic region by the Cretaceous, before going extinct.

The largest extant bony fish is the ocean sunfish (Mola mola), whose average adult weight is 1,000 kg (2,200 lb). While phylogenetically a "bony fish", its skeleton is primarily cartilage (which is lighter than bone). It has a disk-shaped body, and propels itself with its long, thin dorsal and anal fins; it feeds primarily on jellyfish. In these three respects (as well as in its size and diving habits), it resembles a leatherback turtle.

The largest living predatory fish, the great white shark (Carcharodon carcharias), reaches weights up to 2,240 kg (4,940 lb). Its extinct relative C. megalodon (the disputed genus being either Carcharodon or Carcharocles) was more than an order of magnitude larger, and is the largest predatory shark or fish of all time (and possibly the largest predator in vertebrate history); it preyed on whales and other marine mammals.

A number of deep ocean creatures exhibit abyssal gigantism. These include the giant squid (Architeuthis) and colossal squid (Mesonychoteuthis hamiltoni); both (although rarely seen) are believed to attain lengths of 12 m (39 ft) or more. The latter is the world's largest invertebrate, and has the largest eyes of any animal. Both are preyed upon by sperm whales.

Eurypterids (sea scorpions) were a diverse group of aquatic and possibly amphibious predators that included the most massive arthropods to have existed. They survived over 200 million years, but finally died out in the Permian–Triassic extinction event along with trilobites and most other forms of life present at the time, including most of the dominant terrestrial therapsids. The Early DevonianJaekelopterus reached an estimated length of 2.5 m (8.2 ft), not including its raptorialchelicerae, and is thought to have been a freshwater species.

^Nonavian dinosaur size was not similarly constrained because they had a different relationship between body mass and egg size than birds. The 400 kg Aepyornis had larger eggs than nearly all dinosaurs.[19][20]

^Perspective makes the fish appear larger relative to the man standing behind it (another example of a megafaunal species) than it actually is.